1. Field of the Invention
The present invention relates to a control system for a four-wheel drive vehicle and, in particular, to a control system for a four-wheel drive vehicle having differential restricting devices.
2. Description of the Related Art
A four-wheel drive vehicle is, in general, provided with a center differential, mounted between a front propeller shaft and a rear propeller shaft, for compensating the differential, between front wheels and rear wheels, and a rear differential mounted between right and left rear wheels.
For example, Japanese Patent Laid-Open No. 62-166,114 discloses a four-wheel drive vehicle which comprises a front differential, a center differential and a rear differential, each of the differentials including a differential restricting device such as a clutch. The differential restricting device can be operated so as to be in a locking condition or an unlocking condition based on the various traveling conditions of the vehicle such as traveling on a rough road, traveling in a straight line, accelerating condition and traveling in a braking condition, which conditions are determined based on data representing wheel rotating speed and steering angle. The stability, braking performance, acceleration performance and the like of the vehicle can be improved by such operations of the differential restricting device.
Japanese Patent Laid-Open No. 62-166,113 discloses a four-wheel drive vehicle with a center differential restricting device. The center differential restricting device controls a torque to be transmitted based on a differential rotating number between front wheels and rear wheels. The torque is controlled so as to decrease when the differential rotating number between the front wheels and the rear wheels is relatively small. As a result, a good differential operation and a good differential restricting operation of the vehicle can be obtained.
Japanese Patent Laid-Open No. 63-251,327 discloses a four-wheel drive vehicle with a center differential restricting device which is automatically locked when the vehicle is starting. As a result, when the vehicle is starting, a driver does not need to operate manually a center differential so as to be locked, and a differential locking operation can be carried out with certainty.
In the above mentioned conventional four-wheel drive vehicle, the Plural differentials may be locked at the same time by the differential restricting devices. If the plural differentials are locked at the same time, the driving torque changes rapidly and so called torque shock occurs. Moreover, since all differentials are operated to be in locking condition and in unlocking condition under substantially the same conditions at almost same time by the differential restricting devices, hunting phenomena might occur simultaneously in all of the differential restricting devices under a certain driving condition. Furthermore, the respective differential restricting devices are provided with electro-magnetic or fuel-operated multiplate clutches. Where the differential restricting devices are provided with the electro-magnetic multiplate clutches, large electric load is generated in a control system since all electro-magnetic multi-plate clutches are operated simultaneously.